====== AVR3: Using 7-deg display ======
In this scenario, you will implement a code that displays on the 7-segment display.\\

** Prerequisites **\\
You need to book one of the AVR laboratory nodes and ensure the video stream is live.

** Scenario **\\
Implement an algorithm that displays your birth year (or any other 4-digit number) on the LED 7-segment display.

** Result **\\
Observe the number via the video stream.

** Start **\\
Use AVR GCC syntax (as in the instruction): node compilation facilities are preconfigured, and you do not need to build a Makefile; still, it is necessary to follow the exact AVR GCC syntax, e.g., in the case of ''.equ''.\\
Use the function ''display_digit'' that handles displaying a single digit, as provided in the [[..:..:sut|]] to bootstrap your code.

Also note you need to define a stack to use ''delay'' separately (if you use ''rcall'' and ''ret'' instructions). It is obligatory.

In this algorithm, you go "full throttle". You should implement a loop that displays four consecutive digits. At 16 MHz, shield latches responsible for handling the display are close to their limits but still operate fine. Naturally, in a real-world scenario, you would not waste energy implementing it this way. Would you?

** Step 1 **\\
Compose application definitions and configuration. Set up a stack. We do not use the'.section' directive here, but if you plan to use RAM, '.sections' are required and simplify your code.
<code asm>
; --- Single .equ for RAM End ---
.equ SPH,      0x3E     ; Stack Pointer High
.equ SPL,      0x3D     ; Stack Pointer Low
.equ SER_PORT, 0x05     ; PORTB I/O address
.equ SER_PIN,  0        ; GPIO8
.equ DDRD,     0x0A     ; Data Direction Port D
.equ DDRB,     0x04     ; Data Direction Port B
.equ CLK_PORT, 0x0B     ; PORTD I/O address
.equ CLK_PIN,  7        ; GPIO7
.equ LAT_PORT, 0x0B     ; PORTD I/O address
.equ LAT_PIN,  4        ; GPIO4
.equ RAMEND,   0x08FF   ; Arduino Uno memory size

.section .text
.org 0x0000
    rjmp RESET

reset:
    ; Initialise Stack Pointer using hi8 and lo8 functions
    ldi r16, lo8(RAM_END)
    out SPL, r16
    ldi r16, hi8(RAM_END)
    out SPH, r16
</code>

** Step 2 **\\
Configure pins that control the display as outputs (GPIO 8, GPIO 7, GPIO 4):
<code asm>
    ; Initialise display control outputs
    sbi DDRB, SER_PIN  ; Set PB0 as output
    sbi DDRD, CLK_PIN  ; Set PD7 as output
    sbi DDRD, LAT_PIN  ; Set PD4 as output
</code>

** Step 3 **\\
Make a loop that calls the 'display_digit' function four times, varying the numbers and positions. Load arguments. 
As you use constant variables (the easiest, naturally, use ''ldi'' to load registers. Note that digit indexes are zero-based; thus, the leftmost digit is at position 0, and the rightmost is at position 3. 


**Step4**
Copy ''display_digit'' function from the manual to your code and put it after the end of your main loop.
<code asm>
main_loop:
    ; Display Pattern A (D2, D4 ON)
    ldi r16, PATTERN_A
    out PORTB, r16
    rcall delay_2s

    ; Display Pattern B (D1, D3 ON)
    ldi r16, PATTERN_B
    out PORTB, r16
    rcall delay_2s

    rjmp main_loop
</code>

** Result validation **\\
You should be able to see the number on the 7-segment display via the video stream.

** FAQ **\\
When using the printed version of this manual, please refer to the latest online version for the most up-to-date list of FAQs.\\

**It does not show up at all**: Did you compile and upload to the device? Those are separate steps: it is not enough to just compile, but you also need to "flash" the MCU. Also, check your video stream if it "ticks" - the time embedded into the video stream should change. Your code may be working OK, but the video stream can be frozen, so you cannot see it working properly!
//